Mosaic Inlay

Can I use Polymer clay to make my own crystal ball stud like earrings?

I often see these beautiful crystal ball looking earrings and when you look close it looks like there is hardened clay holding the tiny crystals in place. I have even seen the flat ones before with the crystals and the clay is sometimes the color of the actual crystal. I have bought the flat posts and and am just a little unsure about the clay, would I use this?

Sue is right. You can definitely use polymer clay as the basic shape to embed (inlay) "crystals" and other things into. Or you could use air-dry clays, epoxy clays, other claylike materials, etc.

Since polymer clay must be heated to harden, if you leave the inlays in the clay during heating they must be able to tolerate the heat required (but that's fairly low heat...around 275 F). Some items that look like "crystals" are glass or actual crystal minerals, some are plastics of various kinds. The glass and real crystals would be fine, but plastics may soften, melt, discolor, etc, in the range of 200-300 F. The best way to find out would be to do a trial baking with the "crystals" you have.

If they can't take the heat, you'd simply press them into the clay then remove, bake the clay, then glue them back in.
To make that easier when using tiny items like I think you're referring to, glue one of the crystals to the end of a small dowel or rod or tube or even polymer clay rod (back side OUT), then use that to make the impressions for the crystals in the raw clay instead of using the crystals themselves. Polymer clay will not shrink at that size so the holes should stay perfect (if the clay is reasonably firm and you're careful). Or you could perhaps drill divots for each one after baking the clay (with a hand drill**).

Inlaid bits will often stay in polymer clay because they are embedded and partly below the surface, and the clay may well come up around the edges just a tad which will also form a mechanical hold. Check out more on "inlays" of various kinds on this page:
http://glassattic.com/polymer/mosaics_inlay.htm > "Inlays"
If they don't stay thoroughly, you can use various glues when inlaying in the raw clay--like liquid polymer clay or the stronger permanent white glues. Or use various glues if gluing them in after baking--like E6000, 2-part epoxy glues, superglues (only if surfaces match exactly), stronger permanent white glues, etc.
http://glassattic.com/polymer/glues-Diluent.htm
Or you can create various kinds of clay "bezels" around them, etc.

(Polymer clay can be, or be made, virtually any color so color of the clay wouldn't matter to how these are made.)

(Not sure what you mean about "the flat ones"...flat clay, or flat crystals/etc? If flat clay, something like this?
http://www.flickr.com/photos/mandarinmoon/3264439759/in/photostream )

If you want more info about making earrings using metal posts, you might also want to check out this page:
http://glassattic.com/polymer/jewelry.htm > Earrings